Fish scale

{{Short description|Hard skeletal covering of fish}} {{Other uses|Fish scale (disambiguation)}} {{use British English|date=August 2021}} {{use dmy dates|date=August 2021}} [[File:Fish scales.jpg|thumb|right|{{center|Cycloid scales cover these [[teleost fish]] ([[rohu]])}}]]

A '''fish scale''' is a small rigid plate that grows out of the [[skin]] of a fish. The skin of most [[jawed fish]]es is covered with these protective [[scale (zoology)|scale]]s, which can also provide effective [[Underwater camouflage|camouflage]] through the use of [[animal reflectors|reflection]] and [[animal coloration|colouration]], as well as possible hydrodynamic advantages. The term ''scale'' derives from the [[Old French]] {{lang|fro|escale}}, meaning a shell pod or husk.<ref>[https://www.etymonline.com/search?q=scale Scale] ''Etymonline''. Retrieved 28 April 2019.</ref>

Scales vary enormously in size, shape, structure, and extent, ranging from strong and rigid armour plates in fishes such as [[shrimpfish]]es and [[boxfish]]es, to microscopic or absent in fishes such as [[eel]]s and [[anglerfish]]es. The [[morphology (biology)|morphology]] of a scale can be used to identify the species of fish it came from. Scales originated within the jawless [[ostracoderm]]s, ancestors to all jawed fishes today. Most [[Osteichthyes|bony fishes]] are covered with the cycloid scales of [[salmon]] and [[carp]], or the ctenoid scales of [[perch]], or the ganoid scales of [[sturgeon]]s and [[gar]]s. [[Cartilaginous fish]]es ([[shark]]s and [[Batoidea|rays]]) are covered with placoid scales. Some species are covered instead by [[scute]]s, and others have no outer covering on part or all of the skin.

Fish scales are part of the fish's [[integumentary system]], and are produced from the [[mesoderm]] layer of the [[dermis]], which distinguishes them from [[reptile scale]]s.<ref>{{cite journal | last1 = Mongera | first1 = A. | last2 = Nüsslein-Volhard | first2 = C. | year = 2013 | title = Scales of fish arise from mesoderm | journal = Current Biology | volume = 23 | issue = 9| pages = R338–R339 | doi = 10.1016/j.cub.2013.02.056 | pmid = 23660349 | doi-access = free | bibcode = 2013CBio...23.R338M }}</ref><ref>{{Cite journal| first1 = P. T.| title = Fish scale development: Hair today, teeth and scales yesterday?| journal = Current Biology| volume = 11| last1 = Sharpe| issue = 18| pages = R751–R752| year = 2001| pmid = 11566120| doi = 10.1016/S0960-9822(01)00438-9 | s2cid = 18868124| doi-access = free| bibcode = 2001CBio...11.R751S}}</ref> The same [[gene]]s involved in tooth and hair development in [[mammal]]s are also involved in scale development. The placoid scales of cartilaginous fishes are also called dermal denticles and are structurally [[Homology (biology)|homologous]] with vertebrate teeth. Most fish are also covered in a layer of [[mucus]] or slime which can protect against pathogens such as bacteria, fungi, and viruses, and reduce surface resistance when the fish swims.

==Thelodont scales== [[File:Thelodont denticles.png|thumb|left|Left to right: denticles of ''[[Paralogania]]'', ''Shielia taiti'', ''[[Lanarkia]] horrida'']]

The bony scales of [[thelodont]]s, the most abundant form of [[fossil fish]], are well understood. The scales were formed and shed throughout the organisms' lifetimes, and quickly separated after their death.<ref name=Turner1982>{{cite journal|author=Turner, S.|author2=Tarling, D. H. |year=1982 |title=Thelodont and other agnathan distributions as tests of Lower Paleozoic continental reconstructions |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=39 |pages=295–311 |doi=10.1016/0031-0182(82)90027-X|issue=3–4|bibcode=1982PPP....39..295T }}</ref>

Bone, a tissue that is both resistant to mechanical damage and relatively prone to fossilization, often preserves internal detail, which allows the [[histology]] and growth of the scales to be studied in detail. The scales comprise a non-growing "crown" composed of [[dentine]], with a sometimes-ornamented [[enameloid]] upper surface and an aspidine base.<ref name="Märss2006b">{{cite journal | author = Märss, T. | year = 2006 | title = Exoskeletal ultrasculpture of early vertebrates | journal =[[Journal of Vertebrate Paleontology]] | volume = 26 | issue = 2 | pages = 235–252 | doi = 10.1671/0272-4634(2006)26[235:EUOEV]2.0.CO;2| s2cid = 85993241 }}</ref> Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish.<ref name=Janvier1998>{{cite book |author=Janvier, Philippe |title=Early Vertebrates |publisher=[[Oxford University Press]] |year=1998 |isbn=978-0-19-854047-2 |chapter=Early vertebrates and their extant relatives |pages=123–127}}</ref> Beyond that, there appear to be five types of bone growth, which may represent five natural groupings within the thelodonts—or a spectrum ranging between the end members meta- (or ortho-) dentine and mesodentine tissues.<ref name=Turner1991/> Each of the five scale morphs appears to resemble the scales of more derived groupings of fish, suggesting that thelodont groups may have been stem groups to succeeding clades of fish.<ref name=Janvier1998/>

However, using scale morphology alone to distinguish species has some pitfalls. Within each organism, scale shape varies hugely according to body area,<ref name="Märss1986">{{cite journal | doi = 10.1080/02724634.1986.10011593 | author = Märss, T. | year = 1986 | title = Squamation of the thelodont agnathan ''Phlebolepis'' | journal = [[Journal of Vertebrate Paleontology]] | volume = 6 | issue = 1 | pages = 1–11| bibcode = 1986JVPal...6....1M }}</ref> with intermediate forms appearing between different areas—and to make matters worse, scale morphology may not even be constant within one area. To confuse things further, scale morphologies are not unique to taxa, and may be indistinguishable on the same area of two different species.<ref name=Botella2006>{{cite journal | author = Botella, H. |author2=J. I. Valenzuela-Rios |author3=P. Carls | year = 2006 | title = A New Early Devonian thelodont from Celtiberia (Spain), with a revision of Spanish thelodonts |journal=[[Palaeontology (journal)|Palaeontology]] | volume = 49 | issue = 1 | pages = 141–154 | doi = 10.1111/j.1475-4983.2005.00534.x|s2cid=128939911 | doi-access = free |bibcode=2006Palgy..49..141B }}</ref>

The morphology and histology of thelodonts provides the main tool for quantifying their diversity and distinguishing between species, although ultimately using such [[Convergent evolution|convergent]] traits is prone to errors. Nonetheless, a framework comprising three groups has been proposed based upon scale morphology and histology.<ref name=Turner1991>{{Cite book |author=Turner, S. |chapter = Monophyly and interrelationships of the Thelodonti |title=Early Vertebrates and Related Problems of Evolutionary Biology |editor=M. M. Chang |editor2=Y. H. Liu |editor3=G. R. Zhang |pages = 87–119 |publisher = Science Press, Beijing |year = 1991}}</ref> Comparisons to modern shark species have shown that thelodont scales were functionally similar to those of modern cartilaginous fish, and likewise has allowed an extensive comparison between ecological niches.<ref>{{cite journal | last1 = Ferrón | first1 = Humberto G. | last2 = Botella | first2 = Héctor | year = 2017 | title = Squamation and ecology of thelodonts | journal = PLOS ONE | volume = 12 | issue = 2| article-number = e0172781 | doi = 10.1371/journal.pone.0172781 | pmid = 28241029 | pmc = 5328365 | bibcode = 2017PLoSO..1272781F | doi-access = free }}</ref>

==Cosmoid scales== [[File:Barramunda.jpg|thumb|{{center|Queensland lungfish}}]]

Cosmoid scales are found only on ancient [[lobe-finned fish]]es, including some of the earliest [[lungfish]]es (subclass [[Dipnoi]]), and in [[Crossopterygii]], including the living [[coelacanth]] in a modified form (see elasmoid scales, below). They were probably derived from a fusion of placoid-ganoid scales. The inner part of the scales is made of dense [[lamellar]] bone called isopedine. On top of this lies a layer of spongy or [[Blood vessel|vascular]] bone supplied with blood vessels, followed by a complex [[dentine]]-like layer called [[cosmine]] with a superficial outer coating of [[vitrodentine]]. The upper surface is [[keratin]]. Cosmoid scales increase in size through the growth of the lamellar bone layer.<ref>MICHAEL ALLABY "cosmoid scale ." A Dictionary of Zoology . . Encyclopedia.com. 29 Oct. 2019 <https://www.encyclopedia.com></ref>

==Elasmoid scales== [[File:Coelacanth-PaleozoologicalMuseumOfChina-May23-08.jpg|thumb|right|{{center|[[Lobe-finned fish]]es, like this preserved [[coelacanth]], have elasmoid scales.}}]]

Elasmoid scales are thin, [[wikt:imbrication|imbricated]] scales composed of a layer of dense, lamellar collagen bone called isopedine, above which is a layer of tubercles usually composed of bone, as in ''[[Eusthenopteron]]''. The layer of dentine that was present in the first lobe-finned fish is usually reduced, as in the extant [[coelacanth]], or entirely absent, as in extant [[lungfish]] and in the Devonian ''Eusthenopteron''.<ref>Zylberberg, L., Meunier, F.J., Laurin, M. (2010). [http://www.app.pan.pl/article/item/app20091109.html A microanatomical and histological study of the postcranial dermal skeleton in the Devonian sarcopterygian ''Eusthenopteron foordi'', Acta Palaeontologica Polonica] 55: 459–470.</ref> Elasmoid scales have appeared several times over the course of fish evolution. They are present in some [[lobe-finned fish]]es, such as all extant and some extinct lungfishes, as well as the coelacanths which have modified cosmoid scales that lack cosmine and are thinner than true cosmoid scales. They are also present in some tetrapodomorphs like ''Eusthenopteron'', amiids, and teleosts, whose cycloid and ctenoid scales represent the least mineralized elasmoid scales.

The [[zebrafish]] elasmoid scales are used in the lab to study bone mineralization process, and can be cultured (kept) outside of the organism.<ref>{{Cite journal|last1=Bergen|first1=Dylan J. M.|last2=Kague|first2=Erika|last3=Hammond|first3=Chrissy L.|date=2019|title=Zebrafish as an Emerging Model for Osteoporosis: A Primary Testing Platform for Screening New Osteo-Active Compounds|journal=Frontiers in Endocrinology|language=en|volume=10|article-number=6|doi=10.3389/fendo.2019.00006|pmid=30761080|pmc=6361756|issn=1664-2392|doi-access=free}}</ref><ref>{{Cite journal|last1=de Vrieze|first1=E.|last2=van Kessel|first2=M. A. H. J.|last3=Peters|first3=H. M.|last4=Spanings|first4=F. A. T.|last5=Flik|first5=G.|last6=Metz|first6=J. R.|date=2014-02-01|title=Prednisolone induces osteoporosis-like phenotype in regenerating zebrafish scales|journal=Osteoporosis International|language=en|volume=25|issue=2|pages=567–578|doi=10.1007/s00198-013-2441-3|pmid=23903952|s2cid=21829206|issn=1433-2965|hdl=2066/123472|hdl-access=free}}</ref>

==Ganoid scales== [[File:Spotted Gar (Lepisosteus oculatus) (3149758934).jpg|thumb| The scales of this [[spotted gar]] appear glassy due to ganoine.]] [[File:Mineral texture of ganoine layers in the scales of an alligator gar..tif|thumb| Mineral texture of ganoine layers in the scales of an [[alligator gar]]]]

Ganoid scales are found in the [[sturgeon]]s, [[paddlefish]]es, [[gar]]s, [[bowfin]], and [[bichir]]s. They are derived from cosmoid scales and often have serrated edges. They are covered with a layer of hard enamel-like [[dentine]] in the place of [[cosmine]], and a layer of inorganic bone salt called ''[[ganoine]]'' in place of [[vitrodentine]].

Ganoine is a characteristic component of ganoid scales. It is a glassy, often multi-layered mineralized [[tissue (biology)|tissue]] that covers the scales, as well as the [[cranium|cranial]] bones and [[fish fin|fin rays]] in some non-teleost [[Actinopterygii|ray-finned fishes]],<ref name="Zylberberg1997">{{Cite journal | last1 = Zylberberg | first1 = L. | last2 = Sire | first2 = J. -Y. | last3 = Nanci | first3 = A. | doi = 10.1002/(SICI)1097-0185(199709)249:1<86::AID-AR11>3.0.CO;2-X | title = Immunodetection of amelogenin-like proteins in the ganoine of experimentally regenerating scales of Calamoichthys calabaricus, a primitive actinopterygian fish | journal = The Anatomical Record | volume = 249 | issue = 1 | pages = 86–95 | year = 1997 | pmid = 9294653 | doi-access = free }}</ref> such as [[gar]]s, [[bichir]]s, and [[coelacanth]]s.<ref>{{Cite journal|last1=Sire|first1=Jean-Yves|last2=Donoghue|first2=Philip C. J.|last3=Vickaryous|first3=Matthews K.|title=Origin and evolution of the integumentary skeleton in non-tetrapod vertebrates|journal=Journal of Anatomy|language=en|volume=214|issue=4|pages=409–440|doi=10.1111/j.1469-7580.2009.01046.x|issn=0021-8782|pmc=2736117|pmid=19422423|year=2009}}</ref><ref name="Richter1994">{{Cite journal | last1 = Richter | first1 = M. | title = A microstructural study of the ganoine tissue of selected lower vertebrates | doi = 10.1006/zjls.1995.0023 | journal = Zoological Journal of the Linnean Society | volume = 114 | issue = 2 | pages = 173–212 | year = 1995 }}</ref> It is composed of rod-like [[apatite]] crystallites.<ref name="Bruet2008">{{Cite journal | last1 = Bruet | first1 = B. J. F. | last2 = Song | first2 = J. | last3 = Boyce | first3 = M. C. | last4 = Ortiz | first4 = C. | title = Materials design principles of ancient fish armour | doi = 10.1038/nmat2231 | journal = Nature Materials | volume = 7 | issue = 9 | pages = 748–756 | year = 2008 | pmid = 18660814|bibcode = 2008NatMa...7..748B }}</ref> Ganoine is an ancient feature of ray-finned fishes, being found for example on the scales of [[stem group]] actinopteryigian ''[[Cheirolepis]]''.<ref name="Richter1994"/> While often considered a [[synapomorphy|synapomorphic character]] of ray-finned fishes, ganoine or ganoine-like tissues are also found on the extinct [[acanthodii]].<ref name="Richter1994"/> It has been suggested ganoine is [[homology (biology)|homologous]] to [[tooth enamel]] in vertebrates<ref name="Zylberberg1997"/> or even considered a type of enamel.<ref name="Bruet2008"/>

{| class="wikitable" |- | [[File:Amblypterus macropterus.jpg|180px]] {{center|''Amblypterus striatus''}} | width=400px| Ganoid scales of the extinct [[Carboniferous]] fish, ''[[Amblypterus|Amblypterus striatus]]''. (a) shows the outer surface of four of the scales, and (b) shows the inner surface of two of the scales. Each of the rhomboidal-shaped ganoid scales of Amblypterus has a ridge on the inner surface which is produced at one end into a projecting peg which fits into a notch in the next scale, similar to the manner in which tiles are pegged together on the roof of a house. | [[File:Ganoid scales.png|180px]] |}

Most ganoid scales are [[rhomboidal]] (diamond-shaped) and connected by peg-and-socket joints. They are usually thick and fit together more like a jigsaw rather than overlapping like other scales.<ref name=Sherman2016>{{Cite journal|last1=Sherman|first1=Vincent R.|last2=Yaraghi|first2=Nicholas A.|last3=Kisailus|first3=David|last4=Meyers|first4=Marc A.|date=2016-12-01|title=Microstructural and geometric influences in the protective scales of Atractosteus spatula|journal=Journal of the Royal Society Interface|language=en|volume=13|issue=125|article-number=20160595|doi=10.1098/rsif.2016.0595|issn=1742-5689|pmid=27974575|pmc=5221522}}</ref> In this way, ganoid scales are nearly impenetrable and are excellent protection against predation.

<gallery heights="165px" mode="packed" style="float:left;"> File:Alligator gar fish.jpg| The [[alligator gar]] has a tough armouring of [[rhomboidal]]-shaped ganoid scales.<ref name=Sherman2016/> File:Acipenser oxyrhynchus (recropped).png| The [[sturgeon]] has rows of ganoid scales enlarged into [[#Scutes|scute-like]] armour plates. File:Bowfin fish image.jpg| The ganoid scales on a [[bowfin]] are reduced in size and resemble [[cycloid scales]]. </gallery>

[[File:Polypterus bichir from Sudan at Göteborgs Naturhistoriska Museum 9033.jpg|thumb| Geometrically laid out ganoid scales on a [[bichir]]]]

In sturgeons, the scales are greatly enlarged into armour plates along the sides and back, while in the bowfin the scales are greatly reduced in thickness to resemble [[#Cycloid scales|cycloid scales]].

<gallery heights="120px" mode="packed" style="float:left;"> File:Earrings from the ganoid scales of alligator gar.jpg| Earrings made from the ganoid scales of an alligator gar File:Schmelzschuppenfischkonkretion.jpg| Fossil of a primitive rayfin with ganoid scales File:Fish Fossil (FindID 64765).jpg| Ganoid scales on a fossilised ''[[Lepidotes]]'', circa. 130 [[Mya (unit)|mya]] </gallery> {{clear left}}

[[Native Americans in the United States|Native Americans]] and [[Caribbean people|people of the Caribbean]] used the tough ganoid scales of the [[alligator gar]] for arrow heads, breastplates, and as shielding to cover plows. In current times jewellery is made from these scales.<ref name=MDC>{{cite web |url=http://www.sdafs.org/alligar/docs/Missouri%20Alligator%20Gar%20Management%20and%20Restoration%20Plan.pdf |title=Missouri Alligator Gar Management and Restoration Plan|publisher=Missouri Department of Conservation Fisheries Division|date=22 January 2013|access-date=12 April 2019|archive-url=https://web.archive.org/web/20160506052518/http://sdafs.org/alligar/docs/Missouri%20Alligator%20Gar%20Management%20and%20Restoration%20Plan.pdf|archive-date=6 May 2016|df=mdy-all}}</ref>

==Leptoid scales== Leptoid (bony-ridge) scales are found on higher-order bony fish, the [[teleost]]s (the more [[Synapomorphy|derived]] [[clade]] of ray-finned fishes). The outer part of these scales fan out with bony ridges while the inner part is criss-crossed with fibrous connective tissue. Leptoid scales are thinner and more translucent than other types of scales, and lack the hardened enamel-like or dentine layers. Unlike ganoid scales, further scales are added in concentric layers as the fish grows.<ref>Lagler, K. F., J. E. Bardach, and R. R. Miller (1962) ''Ichthyology''. New York: John Wiley & Sons.</ref>

Leptoid scales overlap in a head-to-tail configuration, like roof tiles, making them more flexible than cosmoid and ganoid scales. This arrangement allows a smoother flow of water over the body, and reduces [[Drag (physics)|drag]].<ref>{{cite book | publisher = John Wiley & Sons | isbn = 978-1-118-92421-1| last1 = Ballard| first1 = Bonnie| last2 = Cheek| first2 = Ryan | title = Exotic Animal Medicine for the Veterinary Technician | date = 2 July 2016 | url=https://books.google.com/books?id=h1_NDAAAQBAJ&q=leptoid%20scale&pg=PT786}}</ref> The scales of some species exhibit bands of uneven seasonal growth called ''annuli'' (singular ''annulus''). These bands can be used to [[Identification of aging in fish|age the fish]].

Leptoid scales come in two forms: ''cycloid'' (smooth) and ''ctenoid'' (comb-like).<ref>{{cite web |url=https://australian.museum/learn/animals/fishes/cycloid-and-ctenoid-scales/ |title=Cycloid and Ctenoid Scales |last=McGrouther |first=Mark |date=2 Dec 2019 |website=The Australian Museum |access-date=29 December 2021 }}</ref>

===Cycloid scales=== Cycloid (circular) scales have a smooth texture and are uniform, with a smooth outer edge or margin. They are most common on fish with soft fin rays, such as [[salmon]] and [[carp]].

{| style="border:1px; float:right;" |- | [[File:Gold Arowana035.JPG|200px]] | width=150px | [[File:Asian arowana scales (cropped).jpg|150px]] |- | colspan=2 width=400px | {{center|[[Asian arowana]] have large cycloid scales arranged on the fish in a [[mosaic]] of raised ribs (left). The scales themselves are covered with a delicate net pattern (right).<ref name="Pouyaud">{{cite journal|last=Pouyaud|first=L.|author2=Sudarto, Guy G. Teugels|title=The different colour varieties of the Asian arowana Scleropages formosus (Osteoglossidae) are distinct species: morphologic and genetic evidences|journal=Cybium|year=2003|volume=27|issue=4|pages=287–305}}</ref><ref name="Ismail 1989">{{cite book|last=Ismail|first=M.|title=Systematics, Zoogeography, and Conservation of the Freshwater Fishes of Peninsular Malaysia.|year=1989|publisher=Colorado State University|edition=Doctoral Dissertation}}</ref>}} |}

{{multiple image | align = left | caption_align = center | direction = horizontal | header = Cycloid (circular) scales | header_align = center | header_background = | image1 = PSM V35 D074 Scale of common carp.jpg | width1 = 114 | alt1 = | caption1 = The cycloid scale of a [[common carp|carp]] has a smooth outer edge (at top of image). | width2 = 180 | image2 = Poropuntius huguenini Bleeker.jpg | alt2 = | caption2 = This ''[[Poropuntius huguenini]]'' is a [[Cyprinoid|carp-like]] fish with circular cycloid scales that are smooth to the touch. }}

{| class="wikitable" |- | width=120px| {{center| Cycloid (circular) scales are usually found on carp-like or salmon-like fishes.}} | <gallery mode="packed" heights="60px"> File:FMIB 49568 -Salmon Scale- 40 lbds August 1906 First return from the sea.jpeg| {{centre|[[salmon]]}} File:PSM V35 D072 Scale of bream.jpg| {{centre|[[bream]]}} File:PSM V35 D073 Scale of loach.jpg| {{centre|[[loach]]}} File:PSM V35 D073 Scale of minnow.jpg| {{centre|[[minnow]]}} File:PSM V35 D076 Scale of grayling.jpg| {{centre|[[Thymallus thymallus|grayling]]}} File:PSM V35 D070 Scale of bleak.jpg| {{centre|[[Common Bleak|bleak]]}} File:PSM V35 D069 Scale of chub.jpg| {{centre|[[European chub|chub]]}} File:PSM V35 D075 Scale of pike.jpg| {{centre|[[Northern pike|pike]]}} </gallery> |}

===Ctenoid scales=== Ctenoid (toothed) scales are like cycloid scales, except they have small teeth or [[spinule]]s called '''ctenii''' along their outer or posterior edges. Because of these teeth, the scales have a rough texture. They are usually found on fishes with spiny fin rays, such as the [[Perciformes|perch-like]] fishes. These scales contain almost no bone, being composed of a surface layer containing [[hydroxyapatite]] and [[calcium carbonate]] and a deeper layer composed mostly of [[collagen]]. The enamel of the other scale types is reduced to superficial ridges and ctenii.

{{multiple image | align = left | caption_align = center | direction = horizontal | header = Ctenoid (toothed) scales | header_align = center | header_background = | image1 = PSM V35 D074 Scale of perch.jpg | width1 = 123 | alt1 = | caption1 = The ctenoid scale of a [[perch]] has a toothed outer edge (at top of image). | image2 = Manonichthys splendens.jpg | width2 = 167 | alt2 = | caption2 = This [[dottyback]] is a [[Perciformes|perch-like]] fish with toothed ctenoid scales that are rough to the touch. }}

{| style="border:1px; float:right;" |- | [[File:Cetonurus crassiceps scales.jpg|100px]] | width=300px | [[File:Cetonurus crassiceps2.jpg|300px]] |- | colspan=2 width=400px | {{center|The size of the teeth on ctenoid scales can vary with position, as these scales from the [[rattail]] ''Cetonurus crassiceps'' show.}} |}

[[File:Ctenoid Perch Scales.png|thumb|250px|right|Ctenoid scales from a [[perch]] vary from the medial (middle of the fish), to dorsal (top), to caudal (tail end) scales.]] [[File:Crazy fish, Butis butis (Hamilton, 1822) by M. L. Nievera (colored).png|thumb|250px| [[Crazy fish]] have cycloid scales on the belly but ctenoid scales elsewhere.<ref name="brill">{{cite book|author=E.J. Brill|title =The Fishes of the Indo-Australian Archipelago|publisher =E.J. Brill|year =1953|pages=306–307|url =https://books.google.com/books?id=-4keAAAAIAAJ&q=Eleotris%20butis&pg=PA306}}</ref>]]

{| class="wikitable" |- | width=120px| {{center| Ctenoid (toothed) scales are usually found on perch-like fishes.  }} | <gallery mode="packed" heights="60px"> File:Study of Fishes-Fig 14.png| {{centre|[[goby]]}} File:Study of Fishes-Fig 13.png| {{centre|[[Flathead (fish)|flathead]]}} File:Study of Fishes-Fig 12.png| {{centre|[[Scatophagidae|scat]]}} File:Study of Fishes-Fig 15.png| {{centre|[[Lethrinus|emperor]]}} File:PSM V35 D072 Scale of gudgeon.jpg| {{centre|[[gudgeon (fish)|gudgeon]]}} </gallery> |}

Ctenoid scales, similar to other epidermal structures, originate from [[Neurogenic placodes|placodes]] and distinctive cellular differentiation makes them exclusive from other structures that arise from the [[integument]].<ref name=":0">Kawasaki, Kenta C., "A Genetic Analysis of Cichlid Scale Morphology" (2016). Masters Theses May 2014 - current. 425. http://scholarworks.umass.edu/masters_theses_2/425</ref> Development starts near the [[caudal fin]], along the [[lateral line]] of the fish.<ref>{{Cite book|title=The Diversity of Fishes Biology, Evolution, and Ecology|last=Helfman|first=Gene|publisher=Wiley-Blackwell|year=2009}}</ref> The development process begins with an accumulation of [[fibroblast]]s between the [[epidermis]] and [[dermis]].<ref name=":0" /> [[Collagen fibrils]] begin to organize themselves in the dermal layer, which leads to the initiation of [[Mineralization (biology)|mineralization]].<ref name=":0" /> The circumference of the scales grows first, followed by thickness when overlapping layers mineralize together.<ref name=":0" />

Ctenoid scales can be further subdivided into three types: * '''Crenate''' scales, where the margin of the scale bears indentations and projections. * '''Spinoid''' scales, where the scale bears spines that are continuous with the scale itself. * '''True ctenoid''' scales, where the spines on the scale are distinct structures.

Most ray-finned fishes have ctenoid scales. Some species of [[flatfish]]es have ctenoid scales on the eyed side and cycloid scales on the blind side, while other species have ctenoid scales in males and cycloid scales in females.

===Reflection=== [[File:Herring Silvering.svg|thumb|upright=0.5|left|The herring's reflectors are nearly vertical for camouflage from the side.]]Many teleost fish are covered with highly reflective scales which function as small mirrors and give the appearance of silvered glass. Reflection through silvering is widespread in open ocean fish, especially those that live in the top 100 metres of the ocean. A [[Underwater camouflage#Transparency|transparency]] effect can be achieved by silvering to make an animal's body highly reflective. In fish such as the [[herring]], which lives in shallower water, the mirrors must reflect a mixture of wavelengths, and the fish accordingly has crystal stacks with a range of different spacings. A further complication for fish with bodies that are rounded in cross-section is that the mirrors would be ineffective if laid flat on the skin, as they would fail to reflect horizontally. The overall mirror effect is achieved with many small reflectors, all oriented vertically.<ref name="Herring2002" /> {{Multiple image | direction = vertical | total_width = 260 | image1 = Atlantic herring.jpg | image2 = Hatchetfish - Argyropelecus aculeatus.jpg | caption1 = The scales of many [[teleost]] fish, like this [[Atlantic herring]], are silvered | caption2 = The [[deep sea hatchetfish]] has silvery scales which reflect blue light along the sides of its body }} Below 200 metres, in the [[Mesopelagic zone|mesopelagic]] zone of the ocean, there is only faint blue light, which can still be reflected by mirror-like scales positioned at specific angles along the sides of a fish's body. This results in the fish appearing invisible when viewed from the side, which is termed [[optical camouflage]].<ref name="Herring2002">{{cite book |last1=Herring |first1=Peter |author-link=Peter Herring |year=2002 |title=The Biology of the Deep Ocean |location=Oxford |publisher=Oxford University Press |isbn=978-0-19-854956-7 |pages=193–195}}</ref><ref name=":2">{{Cite book |last=Priede |first=Imants G. |url=https://www.cambridge.org/core/books/deepsea-fishes/88B2EDFF99C5C35DE004BAFDD141080E |title=Deep-Sea Fishes: Biology, Diversity, Ecology and Fisheries |date=2017 |publisher=Cambridge University Press |isbn=978-1-107-08382-0 |location=Cambridge |pages=126–127 |doi=10.1017/9781316018330}}</ref> The backs of these fish are typically covered in dark, non-reflective scales to match the dark water below them, and because light does not reflect off of the underbelly of these fish, they scaleless undersides covered in [[Bioluminescence|bioluminescent]] [[Photophore|photophores]] to provide [[counter-illumination]].<ref name=":2" /> The [[marine hatchetfish]] is extremely flattened laterally (side to side), leaving the body just millimetres thick, and the sides of the body are reflective and silvery. The reflectors consist of microscopic structures similar to those used to provide [[structural coloration]]: stacks of between 5 and 10 [[Crystal|crystals]] of [[guanine]] spaced about ¼ of a wavelength apart to interfere constructively and achieve nearly 100 per cent reflection. In the waters where hatchetfish live, only blue light with a wavelength of 500 nanometres percolates down and needs to be reflected, so mirrors 125 nanometres apart provide good camouflage.<ref name="Herring2002" /> Below 1000 metres in the [[Bathypelagic zone|bathypelagic]] zone there is no sunlight, and reflective optical camouflage is totally ineffective.<ref name=":2" />

Fish scales with these properties are used in some cosmetics, since they can give a shimmering effect to makeup and lipstick.<ref>{{Cite web|url=https://www.huffingtonpost.com/2015/04/23/fish-scales-lipstick_n_7126716.html|title=There Are Probably Fish Scales In Your Lipstick|date=2015-04-23|website=HuffPost India|language=en|access-date=2019-05-06}}</ref>

==Placoid scales== {{Distinguish|Sharkskin}}  [[File:Denticules cutanés du requin citron Negaprion brevirostris vus au microscope électronique à balayage.jpg|thumb|250px|right|Placoid scales as viewed through an electron microscope. Also called dermal denticles, these are structurally homologous with vertebrate teeth.]]

'''Placoid''' (pointed, tooth-shaped) scales are found in the [[cartilaginous fish]]es: [[shark]]s, [[Batoidea|rays]]. They are also called '''dermal denticles'''. Placoid scales are structurally [[homology (biology)|homologous]] with [[vertebrate]] [[tooth (animal)|teeth]] ("denticle" translates to "small tooth"), having a central [[pulp (tooth)|pulp cavity]] supplied with [[blood vessel]]s, surrounded by a conical layer of [[dentine]], all of which sits on top of a rectangular basal plate that rests on the [[dermis]]. The outermost layer is composed of [[vitrodentine]] (also called enameloid), a largely inorganic [[Tooth enamel|enamel]]-like substance. Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size.

Similar scales can also be found under the head of the [[denticle herring]]. The amount of scale coverage is much less in rays.

Rhomboidal scales with the properties of both placoid and ganoid scales are suspected to exist in modern jawed fish ancestors: jawless [[ostracoderms]] and then jawed [[placoderms]].

===Shark skin=== [[File:Tiger shark.jpg|thumb|left|{{center|Cartilaginous fishes, like this [[tiger shark]], have placoid scales (dermal denticles).}}]]

Shark skin is almost entirely covered by small placoid scales. The scales are supported by spines, which feel rough when stroked in a backward direction, but when flattened by the forward movement of water, create tiny [[vortex|vortices]] that reduce [[hydrodynamic]] [[Drag (physics)|drag]] and reduce [[turbulence]], making swimming both more efficient and quieter compared to that of bony fishes.<ref name="SkinTeeth">{{cite web | url = http://www.elasmo-research.org/education/white_shark/scales.htm | title = Skin of the Teeth | first= R. Aidan |last=Martin | access-date = 2007-08-28}}</ref> It also serves a role in [[Biological fouling|anti-fouling]] by exhibiting the [[lotus effect]].<ref>{{Cite journal|last1=Fürstner|first1=Reiner|last2=Barthlott|first2=Wilhelm|last3=Neinhuis|first3=Christoph|last4=Walzel|first4=Peter|date=2005-02-01|title=Wetting and Self-Cleaning Properties of Artificial Superhydrophobic Surfaces|journal=Langmuir|volume=21|issue=3|pages=956–961|doi=10.1021/la0401011|pmid=15667174|issn=0743-7463}}</ref>

All denticles are composed of an interior pulp cavity with a nervous and arterial supply rooted in the [[dermis]] to supply the denticle with mucus.<ref>{{cite journal |title=Structure, biomimetics, and fluid dynamics of fish skin surfaces |year=2016|doi=10.1103/PhysRevFluids.1.060502|s2cid=18118663|last1=Lauder|first1=George V.|last2=Wainwright|first2=Dylan K.|last3=Domel|first3=August G.|last4=Weaver|first4=James C.|last5=Wen|first5=Li|last6=Bertoldi|first6=Katia|journal=Physical Review Fluids|volume=1|issue=6|article-number=060502|bibcode=2016PhRvF...1f0502L|doi-access=free}}</ref> Denticles contain riblet structures that protrude from the surface of the scale; under a microscope this riblet can look like a hook or ridges coming out of the scale. The overall shape of the protrusion from the denticle is dependent on the type of shark and can be generally described with two appearances.<ref name="Dermal Denticles of Three Slowly Sw">{{cite journal |last1=Feld |first1=Katrine |last2=Kolborg |first2=Anne Noer |last3=Nyborg |first3=Camilla Marie |last4=Salewski |first4=Mirko |last5=Steffensen |first5=John Fleng |last6=Berg-Sørensen |first6=Kirstine |title=Dermal Denticles of Three Slowly Swimming Shark Species: Microscopy and Flow Visualization |journal=Biomimetics |date=24 May 2019 |volume=4 |issue=2 |page=38 |doi=10.3390/biomimetics4020038 |pmid=31137624 |pmc=6631580 |issn=2313-7673|doi-access=free }}</ref> The first is a scale in which ridges are placed laterally down the shark and parallel with the flow of the water. The second form is a smooth scale with what looks like a hooked riblet curling out of the surface aiming towards the [[Anatomical terms of location|posterior side]] of the shark.<ref name="Dermal Denticles of Three Slowly Sw"/> Both riblet shapes assist in creating a turbulent [[boundary layer]] forcing the [[laminar flow]] farther away from the sharks skin.<ref>{{cite journal |last1=Fletcher |first1=Thomas |last2=Altringham |first2=John |last3=Peakall |first3=Jeffrey |last4=Wignall |first4=Paul |last5=Dorrell |first5=Robert |title=Hydrodynamics of fossil fishes |journal=Proceedings of the Royal Society B: Biological Sciences |date=7 August 2014 |volume=281 |issue=1788 |article-number=20140703 |doi=10.1098/rspb.2014.0703 |pmid=24943377 |pmc=4083790 |bibcode=2014PBioS.28140703F |issn=0962-8452}}</ref>

Unlike bony fish, sharks have a complicated dermal corset made of flexible [[collagen]]ous [[fiber]]s arranged as a [[helix|helical]] network surrounding their body. The corset works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy.<ref name="Cartilagious">{{cite web|url=http://www.elasmo-research.org/education/topics/p_cartilage.htm|title=The Importance of Being Cartilaginous|last=Martin|first=R. Aidan|publisher=ReefQuest Centre for Shark Research|access-date=2009-08-29}}</ref> Depending on the position of these placoid scales on the body, they can be flexible and can be passively erected, allowing them to change their angle of attack. These scales also have riblets which are aligned in the direction of flow, these riblets reduce the drag force acting on the shark skin by pushing the vortex further away from the skin surface, inhibiting any high-velocity cross-stream flow.<ref name="Hage 403–412">{{Cite journal|last1=Hage|first1=W.|last2=Bruse|first2=M.|last3=Bechert|first3=D. W.|date=2000-05-01|title=Experiments with three-dimensional riblets as an idealized model of shark skin|journal=Experiments in Fluids|language=en|volume=28|issue=5|pages=403–412|doi=10.1007/s003480050400|issn=1432-1114|bibcode=2000ExFl...28..403B|s2cid=122574419}}</ref>

====Scale morphology==== The general anatomy of the scales varies, but all of them can be divided into three parts: the crown, the neck and the base. The scale pliability is related to the size of the base of the scale. The scales with higher flexibility have a smaller base, and thus are less rigidly attached to the ''stratum laxum.'' On the crown of the fast-swimming sharks there are a series of parallel riblets or ridges which run from an anterior to posterior direction.<ref name="Motta 1096–1110">{{Cite journal|last1=Motta|first1=Philip|last2=Habegger|first2=Maria Laura|last3=Lang|first3=Amy|last4=Hueter|first4=Robert|last5=Davis|first5=Jessica|date=2012-10-01|title=Scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and the blacktip shark Carcharhinus limbatus|journal=Journal of Morphology|language=en|volume=273|issue=10|pages=1096–1110|doi=10.1002/jmor.20047|pmid=22730019|bibcode=2012JMorp.273.1096M |s2cid=23881820|issn=1097-4687}}</ref>

Analyzing the three components of the scale it can be concluded that the base of the denticle does not come into contact with any portion of the fluid flow.<ref name="Bionic Research on Fish Scales for">{{cite journal |last1=Dou |first1=Zhaoliang |last2=Wang |first2=Jiadao |last3=Chen |first3=Darong |title=Bionic Research on Fish Scales for Drag Reduction |journal=Journal of Bionic Engineering |date=1 December 2012 |volume=9 |issue=4 |pages=457–464 |doi=10.1016/S1672-6529(11)60140-6 |s2cid=137143652 |url=https://www.sciencedirect.com/science/article/pii/S1672652911601406 |issn=1672-6529|url-access=subscription }}</ref> The crown and the neck of the denticles however play a key role and are responsible for creating the turbulent vortices and [[Eddy (fluid dynamics)|eddies]] found near the skin's surface.<ref name="Bionic Research on Fish Scales for"/> Because denticles come in so many different shapes and sizes, it can be expected that not all shapes will produce the same type of [[Turbulence|turbulent flow]]. During a recent research experiment [[Biomimetics|biomimetic]] samples of shark denticles with a crescent like microstructure were tested in a water tank using a traction table as a slide. The experiment showed that the surface with denticles experienced a 10% drag reduction overall versus the smooth sample. The reason for this drag reduction was that the turbulent vortices became trapped between the denticles, creating a 'cushion like' barrier against the laminar flow.<ref>{{cite web |title=Experimental investigations on drag-reduction characteristics of bionic surface with water-trapping microstructures of fish scales |url=https://www.nature.com/articles/s41598-018-30490-x.pdf}}</ref> This same type of experiment was performed by another research group which implemented more variation in their biomimetic sample. The second group arrived at the same conclusion as the first. However, because their experiment contained more variation within the samples they were able to achieve a high degree of experimental accuracy. In conclusion, they stated that more practical shapes were more durable than ones with intricate ridge-lines. The practical shapes were low profile and contained trapezoidal or semi-circular trough-like cross sections, and were less effective but nonetheless reduced drag by 6 or 7%.<ref>{{cite journal |last1=Palmer |first1=Colin |last2=Young |first2=Mark T. |title=Surface drag reduction and flow separation control in pelagic vertebrates, with implications for interpreting scale morphologies in fossil taxa |journal=Royal Society Open Science |date=14 January 2015 |volume=2 |issue=1 |article-number=140163 |doi=10.1098/rsos.140163 |doi-access=free|pmid=26064576 |pmc=4448786 |issn=2054-5703|bibcode=2015RSOS....240163P }}</ref>

====Drag reduction==== [[File:Boundary Layer Fr.svg|thumb|right|Effects of turbulent flow on boundary layer]] [[File:SeparationBubble.jpg|thumb|Diagram of the side profile of a shark denticle showing a vortex in the wake downstream of the denticle]] Sharks decrease drag and overall [[cost of transport]] (COT) through multiple different avenues. [[Pressure drag]] is created from the pressure difference between the anterior and posterior sides of the shark due to the amount of volume that is pushed past the shark to propel itself forward.<ref name="Structure, biomimetics, and fluid d">{{cite journal |last1=Lauder |first1=George V. |last2=Wainwright |first2=Dylan K. |last3=Domel |first3=August G. |last4=Weaver |first4=James C. |last5=Wen |first5=Li |last6=Bertoldi |first6=Katia |title=Structure, biomimetics, and fluid dynamics of fish skin surfaces |journal=Physical Review Fluids |date=18 October 2016 |volume=1 |issue=6 |article-number=060502 |doi=10.1103/PhysRevFluids.1.060502 |bibcode=2016PhRvF...1f0502L |doi-access=free }}</ref> This type of drag is also directly proportional to the [[laminar flow]]. When the laminar flow increases around the fish the pressure drag does as well.<ref>{{cite journal |last1=Muthuramalingam |first1=Muthukumar |last2=Villemin |first2=Leo S. |last3=Bruecker |first3=Christoph |title=Streak formation in flow over Biomimetic Fish Scale Arrays |journal=The Journal of Experimental Biology |date=29 April 2019 |volume=222 |issue=Pt 16 |pages=jeb205963 |doi=10.1242/jeb.205963 |pmid=31375542 |language=en|arxiv=1904.12752 |bibcode=2019arXiv190412752M |s2cid=139103148 }}</ref> Frictional drag is a result of the interaction between the fluid against the shark's skin and can vary depending on how the boundary layer changes against the surface of the fish.<ref name="Structure, biomimetics, and fluid d"/>

The riblets impede the cross-stream translation of the streamwise vortices in the viscous sublayer. The mechanism is complex and not yet understood fully. Basically, the riblets inhibit the vortex formation near the surface because the vortex cannot fit in the valleys formed by the riblets. This pushes the vortex further up from the surface, interacting only with the riblet tips, not causing any high-velocity flow in the valleys. Since this high-velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too.<ref name="Motta 1096–1110"/>

Recent research has shown that there is a pre and post-breakdown regime in the near-wall boundary layer where the [[Laminar sublayer|sublayer]] thickens at a declining rate and then abruptly undergoes a breakdown into turbulent vortices before finally collapsing. This system is completely self-regulating and mediates the growth and decay cycle; the vortices accumulate during the growth period and are abruptly liquidated into [[Strouhal number|Strouhal arrays]] of hairpin vortices lifting off the wall. Lifting vortices are what push the boundary layer out and away from the surface of the shark which results in reducing the overall drag experienced by the fish.<ref>{{cite journal |last1=Bandyopadhyay |first1=Promode R. |last2=Hellum |first2=Aren M. |title=Modeling how shark and dolphin skin patterns control transitional wall-turbulence vorticity patterns using spatiotemporal phase reset mechanisms |journal=Scientific Reports |date=23 October 2014 |volume=4 |article-number=6650 |doi=10.1038/srep06650 |pmid=25338940 |pmc=4206846 |language=en |issn=2045-2322|bibcode=2014NatSR...4.6650B}}</ref>

===Technical application=== The rough, [[sandpaper]]-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide [[leather]], called [[shagreen]]. One of the many historical applications of shark shagreen was in making hand-grips for [[sword]]s. The rough texture of the skin is also used in [[Japanese cuisine]] to make [[grater]]s called ''[[oroshigane|oroshiki]]'', by attaching pieces of shark skin to wooden boards. The small size of the scales grates the food very finely.

[[File:Barnacles on Barbette boat. Spain. 1975 (37498295600).jpg|thumb|right|Barnacle growth on boat hull]] In the marine industry, fouling is the process by which something in the water becomes encrusted with sea life such as [[barnacle]]s and [[algae]]. When ships' hulls are fouled, they are much less efficient (because they are rougher), and they are expensive and time-consuming to clean. Therefore, inexpensive and environmentally safe [[Biofouling|anti-fouling surfaces]] are in very high demand to increase the efficiency of shipping, fishing, and naval fleets, among other applications. Dermal denticles are a promising area of research for this type of application due to the fact that sharks are among the only fish without build up or growth on their scales. Studies by the [[United States Navy|U.S. Navy]] have shown that if a biomimetic material can be engineered, it could potentially lead to fuel cost savings for military vessels of up to 45%.<ref>{{cite journal |last1=Magin |first1=Chelsea M. |last2=Cooper |first2=Scott P. |last3=Brennan |first3=Anthony B. |title=Non-toxic antifouling strategies |journal=Materials Today |date=1 April 2010 |volume=13 |issue=4 |pages=36–44 |doi=10.1016/S1369-7021(10)70058-4 |issn=1369-7021|doi-access=free }}</ref>

There are many examples of [[biomimetic material]]s and surfaces based on the structure of aquatic organisms, including sharks. Such applications intend to enable more efficient movement through fluid mediums such as air, water, and oil.

Surfaces that mimic the skin of sharks have also been used in order to keep microorganisms and [[algae]] from coating the hulls of submarines and ships. One variety is traded as "[[Sharklet (material)|sharklet]]".<ref name="sciencedirect.com">{{Cite journal|date=2012-12-15|title=A new method for producing "Lotus Effect" on a biomimetic shark skin|journal=Journal of Colloid and Interface Science|language=en|volume=388|issue=1|pages=235–242|doi=10.1016/j.jcis.2012.08.033|pmid=22995249|issn=0021-9797|last1=Liu|first1=Yunhong|last2=Li|first2=Guangji|bibcode=2012JCIS..388..235L}}</ref><ref>{{Cite web|url=https://www.sharklet.com/our-technology/sharklet-discovery/|title=Sharklet Discovery {{!}} Sharklet Technologies, Inc.|website=www.sharklet.com|language=en-US|access-date=2018-09-26|archive-date=31 July 2020|archive-url=https://web.archive.org/web/20200731161232/https://www.sharklet.com/our-technology/sharklet-discovery/}}</ref>

A lot of the new methods for replicating shark skin involve the use of [[polydimethylsiloxane]] (PDMS) for creating a mold. Usually the process involves taking a flat piece of shark skin, covering it with the PDMS to form a mold and pouring PDMS into that mold again to get a shark skin replica. This method has been used to create a biomimetic surface which has [[Superhydrophobic coating|superhydrophobic]] properties, exhibiting the [[lotus effect]].<ref name="sciencedirect.com"/> One study found that these biomimetic surfaces reduced drag by up to 9%,<ref name="Hage 403–412"/> while with flapping motion drag reduction reached 12.3%.<ref>{{Cite journal|last1=Lauder|first1=George V.|last2=Oeffner|first2=Johannes|date=2012-03-01|title=The hydrodynamic function of shark skin and two biomimetic applications|journal=Journal of Experimental Biology|language=en|volume=215|issue=5|pages=785–795|doi=10.1242/jeb.063040|issn=1477-9145|pmid=22323201|doi-access=free|bibcode=2012JExpB.215..785O }}</ref>

Denticles also provide drag reduction on objects where the main form of drag is caused by turbulent flow at the surface. A large portion of the total drag on long objects with relatively flat sides usually comes from turbulence at the wall, so riblets will have an appreciable effect. Along with marine applications, the aerospace industry can benefit greatly from these biomimetic designs. Other applications include pipes, where they score the insides to a riblet-like roughness and have discovered a 5% drag reduction, and a few percent reduction is claimed with competitive swimwear.<ref name="Dean, Brian 2010">Dean, Brian & Bhushan, Bharat. (2010). Shark-Skin Surfaces for Fluid-Drag Reduction in Turbulent Flow: A Review. Philosophical transactions. Series A, Mathematical, physical, and engineering sciences. 368. 4775-806. 10.1098/rsta.2010.0201.</ref>

[[Parametric model]]ing has been done on shark denticles with a wide range of design variations such as low and high-profile vortex generators.<ref name="Shark skin-inspired designs that im">{{cite journal |last1=Domel |first1=August G. |last2=Saadat |first2=Mehdi |last3=Weaver |first3=James C. |last4=Haj-Hariri |first4=Hossein |last5=Bertoldi |first5=Katia |last6=Lauder |first6=George V. |title=Shark skin-inspired designs that improve aerodynamic performance |journal=Journal of the Royal Society Interface |date=28 February 2018 |volume=15 |issue=139 |article-number=20170828 |doi=10.1098/rsif.2017.0828 |pmid=29436512 |pmc=5832729 }}</ref> Through this method, the most thorough characterization has been completed for symmetrical two-dimensional riblets with sawtooth, scalloped and blade cross sections.<ref name="Dean, Brian 2010"/> These biomimetic models were designed and analyzed to see the effects of applying the denticle-like structures to the wings of various airplanes. During the simulation, it was noted that the sample altered how the low and high [[Angle of attack|angles of attack]] reacted. Both the geometry of the denticles and their arrangement have a profound effect on the aerodynamic response of the aerofoils. Out of both the low and high-profile samples tested, the low-profile vortex generators outperformed the current smooth wing structures by 323%. This increase in performance is due to a separation bubble in the denticle's wake and stream-wise vortices that replenish momentum lost in the boundary layer due to skin friction.<ref name="Shark skin-inspired designs that im"/>

==Scutes== [[File:FMIB 51542 Pine-cone fish, Monocentris japonicus (Houttnyn) Waka, Japan.jpeg|thumb| {{center|[[Pineconefish]] are covered in scutes.}}]]

[[Scute]]s are similar to scales and serve the same function. Unlike the scales of fish, which are formed from the epidermis, scutes are formed in the lower vascular layer of the skin and the epidermal element is only the top surface. Forming in the living dermis, the scutes produce a horny outer layer, that is superficially similar to that of scales.

Scute comes from Latin for ''shield'', and can take the form of: * an external shield-like bony plate, or * a modified, thickened scale that often is keeled or spiny, or * a projecting, modified (rough and strongly ridged) scale, usually associated with the lateral line, or on the caudal peduncle forming caudal keels, or along the ventral profile.

Some fish, such as [[pineconefish]], are completely or partially covered in scutes. [[Alosa|River herrings]] and [[threadfin]]s have an abdominal row of scutes, which are scales with raised, sharp points that are used for protection. Some [[Carangidae|jacks]] have a row of scutes following the [[lateral line]] on either side.

==Scale development== Scales typically appear late in the development of fish. In the case of [[zebrafish]], it takes 30 days after fertilization before the different layers needed to start forming the scales have differentiated and become organized. For this it is necessary that consolidation of the [[mesenchyme]] occurs, then [[morphogenesis]] is induced, and finally the process of differentiation or late [[metamorphosis]] occurs.<ref name=Sire2003>{{cite journal | last1 = Sire | first1 = J.Y. | last2 = Huysseune | first2 = A.N.N. | s2cid = 19556201 | year = 2003 | title = Formation of dermal skeletal and dental tissues in fish: a comparative and evolutionary approach | journal = Biological Reviews | volume = 78 | issue = 2| pages = 219–249 | doi = 10.1017/S1464793102006073 | pmid = 12803422 }}</ref><ref name=Guellec2004>{{cite journal | last1 = Le Guellec | first1 = D. | last2 = Morvan-Dubois | first2 = G. | last3 = Sire | first3 = J.Y. | year = 2004 | title = Skin development in bony fish with particular emphasis on collagen deposition in the dermis of the zebrafish (''Danio rerio'') | url = http://www.ijdb.ehu.es/web/paper/15272388/skin-development-in-bony-fish-with-particular-emphasis-on-collagen-deposition-in-the-dermis-of-the-zebrafish-danio-rerio | journal = International Journal of Developmental Biology | volume = 48 | issue = 2–3| pages = 217–231 | doi = 10.1387/ijdb.15272388 | pmid = 15272388 | doi-access = free }}</ref>

* Mesenchyme consolidation: The consolidation or structuring of the mesenchyme originates during the development of the [[dermis]]. This process depends on whether the fish is cartilaginous or bony. For cartilaginous fish the structuring originates through the formation of two layers. The first is superficial and wide and the second is thin and compact. These two layers are separated by [[mesenchymal cell]]s. Bony fish generate an [[acellular]] substrate organized by perpendicularly by [[collagen fiber]]s. Subsequently, for both fish the [[fibroblast]]s elongate. These penetrate the compact layer of the mesenchyme, which consolidates prior to the formation of the scale, in order to initiate the dermal plate.<ref name=Sire2003 /><ref name=Guellec2004 /><ref name=Sire2001>{{cite journal | last1 = Sire | first1 = J.Y. | year = 2001 | title = Teeth outside the mouth in teleost fishes: how to benefit from a developmental accident | journal = Evolution & Development | volume = 3 | issue = 2| pages = 104–108 | doi = 10.1046/j.1525-142x.2001.003002104.x | pmid = 11341672 | s2cid = 13353402 }}</ref> * Morphogenesis induction: The morphogenesis is due to the formation of the epidermal [[Papilla (fish anatomy)|papilla]], which is generated by joining the [[epidermis]] and [[dermis]] through a process of [[invagination]]. Morphogenesis begins at the time when fibroblasts are relocated to the upper part of the compact mesenchyme. Throughout this process, the [[Stratum basale|basal cell]]s of the [[epithelium]] form a delimiting layer, which is located in the upper part of the mesenchyme. Subsequently, these cells will differentiate in the area where the scale [[primordium]] will arise.<ref name=Sire2003 /><ref name=Guellec2004 /><ref name=Sire2001/> * Differentiation or late metamorphosis: This differentiation is generated by two different forms according to the type of scale being formed. The formation of elasmoid scales (cycloids and ctenoids) occurs through the formation of a space between the matrix of the epidermal papilla. This space contains collagen fibers. Around this space elasmoblasts differentiate and are responsible for generating the necessary material for the formation of the scale. Subsequently, [[Mineralized tissue|matrix mineralization]] occurs, allowing the scale to acquire the rigid characteristic that identifies them.<ref name=Sire2003 /><ref name=Guellec2004 /><ref name=Sire2001/>

Unlike elasmoid scales, [[#Ganoid scales|ganoid scales]] are composed of mineralized and non-mineralized collagen in different regions. The formation of these occurs through the entry of the surface cells of the mesenchyme into the matrix, the latter is composed of collagen fibers and is located around the vascular capillaries, thus giving rise to vascular cavities. At this point, elasmoblasts are replaced by [[osteoblast]]s, thus forming bone. The patches of the matrix of the scale that are not [[ossified]] are composed of compacted collagen that allow it to maintain the union with the mesenchyme. This are known as [[Sharpey fibers]].<ref name=Sire2003 /><ref name=Guellec2004 /><ref name=Sire2001/>

One of the genes that regulate the development of scale formation in fish is the [[sonic hedgehog]] (shh) gene, which by means of the (shh) protein, involved in [[organogenesis]] and in the process of [[cellular communication]], enable the formation of the scales.<ref name=Sire2003b>{{cite journal | last1 = Sire | first1 = J.Y. | last2 = Akimenko | first2 = M.A. | year = 2003 | title = Scale development in fish: a review, with description of sonic hedgehog (shh) expression in the zebrafish (''Danio rerio'') | journal = International Journal of Developmental Biology | volume = 48 | issue = 2–3| pages = 233–247 | doi = 10.1387/ijdb.15272389 | pmid = 15272389 | doi-access = free }}</ref><ref name=Monnot1999>{{cite journal | last1 = Monnot | first1 = M.J. | last2 = Babin | first2 = P.J. | last3 = Poleo | first3 = G. | last4 = Andre | first4 = M. | last5 = Laforest | first5 = L. | last6 = Ballagny | first6 = C. | last7 = Akimenko | first7 = M.A. | year = 1999 | title = Epidermal expression of apolipoprotein E gene during fin and scale development and fin regeneration in zebrafish | journal = Developmental Dynamics | volume = 214 | issue = 3| pages = 207–215 | doi = 10.1002/(SICI)1097-0177(199903)214:3<207::AID-AJA4>3.0.CO;2-5 | pmid = 10090147 | doi-access = free }}</ref> The [[apolipoprotein E]] (ApoE), that allows the transport and metabolism of [[triglyceride]]s and [[cholesterol]], has an interaction with shh, because ApoE provides cholesterol to the [[Hedgehog signaling pathway|shh signaling pathway]]. It has been shown that during the process of [[cell differentiation]] and [[Cell–cell interaction|interaction]], the level of ApoE transcription is high, which has led to the conclusion that this protein is important for the late development of scales.<ref name=Sire2003b/><ref name=Monnot1999/>

==Modified scales== {{multiple image | align = right | direction = horizontal | header = Lateral line | header_align = center | image1 = Scale Common Roach.JPG | width1 = 162 | alt1 = | caption1 = Cycloid scales of a [[common roach]]. Modified scales along the lateral line are visible in the lower half. | width2 = 178 | image2 = Study of Fishes-Fig 20.png | alt2 = | caption2 = {{center|Closeup of a modified cycloid scale from the lateral line of a [[wrasse]]}} }}

{{multiple image | align = right | direction = horizontal | header = Surgeonfish | header_align = center | footer = Surgeonfish (left) have a sharp, scalpel-like modified scale on either side just before the tail. Closeup (right). | footer_align = center | footer_background = | background color = | image1 = Acanthurus dussumieri 2RLS.jpg | width1 = 160 | alt1 = | caption1 = | image2 = Acanthurus spine peduncle.jpg | width2 = 178 | alt2 = | caption2 = }}

Different groups of fish have [[evolved]] a number of modified scales to serve various functions.

* Almost all fishes have a [[lateral line]], a system of [[mechanoreceptor]]s that detect water movements. In bony fishes, the scales along the lateral line have central pores that allow water to contact the sensory cells. * The dorsal fin spines of [[Squaliformes|dogfish shark]]s and chimaeras, the stinging tail spines of [[stingrays]], and the "saw" teeth of [[sawfish]]es and [[sawshark]]s are fused and modified placoid scales. * [[Surgeonfish]] have a scalpel-like blade, which is a modified scale, on either side of the [[caudal peduncle]].<ref>{{cite journal | last1 = Sorenson | first1 = L. | last2 = Santini | first2 = F. | last3 = Carnevale | first3 = G. | last4 = Alfaro | first4 = M.E. | year = 2013 | title = A multi-locus timetree of surgeonfishes (Acanthuridae, Percomorpha), with revised family taxonomy | journal = Molecular Phylogenetics & Evolution | volume = 68 | issue = 1| pages = 150–160 | doi = 10.1016/j.ympev.2013.03.014 | pmid = 23542000 | bibcode = 2013MolPE..68..150S }}</ref> * Some [[herring]]s, [[anchovies]], and [[halfbeak]]s have '''deciduous scales''', which are easily shed and aid in escaping predators. * Male ''[[Percina]]'' darters have a row of enlarged '''caducous scales''' between the [[pelvic fin]]s and the [[anus]]. * [[Porcupine fish]]es have scales modified into large external [[spine (zoology)|spine]]s. * By contrast, [[pufferfish]] have thinner, more hidden spines than porcupine fish, which become visible only when the fish puffs up. Unlike the porcupine fish, these spines are not modified scales, but develop under the control of the same network of genes that produce feathers and hairs in other vertebrates.<ref>[https://phys.org/news/2019-07-pufferfish-wacky-spines.html How the pufferfish got its wacky spines] ''Phys.org'', 25 July 2019.</ref><ref>{{cite journal | last1 = Shono | first1 = T. | last2 = Thiery | first2 = A.P. | last3 = Cooper | first3 = R.L. | last4 = Kurokawa | first4 = D. | last5 = Britz | first5 = R. | last6 = Okabe | first6 = M. | last7 = Fraser | first7 = G.J. | year = 2019 | title = Evolution and Developmental Diversity of Skin Spines in Pufferfishes | journal = iScience | volume = 19| pages = 1248–1259| doi = 10.1016/j.isci.2019.06.003 | pmid = 31353167 | pmc = 6831732 | bibcode = 2019iSci...19.1248S }}</ref>

<gallery mode="packed" heights="110px" style="float:left;"> File:FMIB 45807 Atopomycterus nichthemerus.jpeg| [[Porcupine fish]] have scales modified into [[spine (zoology)|spine]]s. File:Tetrodon patoca Achilles 182.jpg| [[Pufferfish]] spines are not modified scales but are developed by an independent gene network. </gallery> {{Clear}}

==Fish without scales== <gallery mode="packed" heights="200px" style="float:right;"> File:Synchiropus splendidus 2 Luc Viatour cropped.png| [[Synchiropus splendidus|Mandarinfish]] lack scales and protect themselves with a layer of smelly and bitter slime. </gallery>

Fish without scales usually evolve alternatives to the protection scales can provide, such as tough leathery skin or bony plates.

* [[Jawless fish]] ([[lamprey]]s and [[hagfish]]es) have smooth skin without scales and without dermal bone.<ref>Coolidge E, Hedrick MS and Milsom WK (2011) [https://books.google.com/books?id=gfBc_omOIeAC&q=%22Fish+Physiology%3A+Primitive+Fishes%22 "Ventilatory Systems"]. In: McKenzie DJ, Farrell AP and Brauner CJ (Eds) ''Fish Physiology: Primitive Fishes'', Elsevier, Page 182–213. {{ISBN|9780080549521}}</ref> Lampreys get some protection from a tough leathery skin. Hagfish exude copious quantities of slime or [[mucus]] if they are threatened.<ref name="r2">{{cite news | last=Rothschild | first=Anna | title=Hagfish slime: The clothing of the future? | url= https://www.bbc.co.uk/news/magazine-21954779 | work=BBC News | date= 2013-04-01 | access-date=2013-04-02}}</ref> They can tie themselves in an [[overhand knot]], scraping off the slime as they go and freeing themselves from a predator.<ref name=":1">{{Cite web|url=https://www.theatlantic.com/science/archive/2019/01/hagfish-slime/581002/|title=No One Is Prepared for Hagfish Slime|last=Yong|first=Ed|date=2019-01-23|website=The Atlantic|language=en-US|access-date=2019-01-26}}</ref> * Most [[eel]]s are scaleless, though some species are covered with tiny smooth cycloid scales. * Most [[catfish]] lack scales, though several families have body armour in the form of dermal plates or some sort of scute.<ref>{{cite journal|author1=Friel, J P |author2=Lundberg, J G |year=1996|title=''Micromyzon akamai'', gen. et sp. nov., a small and eyeless banjo catfish (Siluriformes: Aspredinidae) from the river channels of the lower Amazon basin|journal=[[Copeia]]|issue=3|pages=641–648|jstor=1447528|volume=1996|doi=10.2307/1447528}}</ref> * [[Synchiropus splendidus|Mandarinfish]] lack scales and have a layer of smelly and bitter slime which blocks out disease and probably discourages predators, implying their bright coloration is [[aposematic]].<ref>{{cite journal|last1=Sadovy|first1=Y.|last2=Randall|first2=J. E.|last3=Rasotto|first3=Maria B.|title=Skin structure in six dragonet species (Gobiesociformes; Callionymidae): Interspecific differences in glandular cell types and mucus secretion|journal=Journal of Fish Biology|date=May 2005|volume=66|issue=5|pages=1411–1418|doi=10.1111/j.0022-1112.2005.00692.x|bibcode=2005JFBio..66.1411S }}</ref> * [[Anglerfish]] have loose, thin skin often covered with fine forked dermal prickles or [[tubercle]]s, but they do not have regular scales. They rely on camouflage to avoid the attention of predators, while their loose skin makes it difficult for predators to grab them.

Many groups of bony fishes, including [[pipefish]], [[seahorse]]s, [[boxfish]], [[Poacher (fish)|poachers]], and several families of [[stickleback]]s, have developed external bony plates, structurally resembling placoid scales, as protective armour against predators. * Seahorses lack scales but have thin skin stretched over a bony plate armour arranged in rings through the length of their bodies. * In boxfish, the plates fuse together to form a rigid shell or [[exoskeleton]] enclosing the entire body. These bony plates are not modified scales but skin that has been [[Ossification|ossified]]. Because of this heavy armour boxfish are limited to slow movements, but few other fish are able to eat the adults.

{| style="border:1px; float:right;" |- | [[File:PSM V35 D070 Scale of eel.jpg|60px]] | width=220px | [[File:Anguilla japonica 1856.jpg|220px]] |- | colspan=2 width=220 | {{center| [[Eel]]s seem scaleless, but some species are covered with tiny smooth cycloid scales.}} |}

<gallery mode="packed" heights="120px" style="float:left;"> File:Lactoria fornasini1.jpg| [[Boxfish]] have plates of ossified skin fused together to form a rigid shell. File:Hippocampus guttulatus Achilles 174.jpg| [[Seahorse]]s have thin skin stretched over bony plates arranged in rings. </gallery> {{Clear}}

Some fish, such as hoki and swordfish, are born with scales but shed them as they grow.

Filefish have rough non-overlapping scales with small spikes, which is why they are called filefish. Some filefish appear scaleless because their scales are so small.

Prominent scaling appears on [[tuna]] only along the lateral line and in the corselet, a protective band of thickened and enlarged scales in the shoulder region. Over most of their body tuna have scales so small that to casual inspection they seems scaleless.<ref>[https://www.nefsc.noaa.gov/faq/faq-archive/fishfaq1d.html Do tunas have scales?] Northeast Fisheries Science Center, NOAA Fisheries. Accessed 4 August 2019.</ref>

<gallery mode="packed" heights="120px" style="float:left;"> File:Cantherhines fronticinctus.jpg| Some [[filefish]] appear scaleless because their scales are so small. File:Bluefin-big.jpg| To casual examination [[tuna]] seem largely free of scales, but they are not. </gallery> {{Clear}}

==Lepidophagy== [[File:Dorsal view of right-bending and left-bending mouth morphs of the cichlid Perissodus microlepis - journal.pone.0044670.g001.png|thumb|Dorsal view of right-bending (left) and left-bending (right) jaw morphs adapted for eating fish scales<ref name=Lee2012>{{cite journal | last1 = Lee | first1 = H. J. | last2 = Kusche | first2 = H. | last3 = Meyer | first3 = A. | date = 2012 | title = Handed Foraging Behavior in Scale-Eating Cichlid Fish: Its Potential Role in Shaping Morphological Asymmetry | journal = PLOS ONE | volume = 7 | issue = 9| article-number = e44670 | doi = 10.1371/journal.pone.0044670 | pmid=22970282 | pmc=3435272| bibcode = 2012PLoSO...744670L | doi-access = free }}</ref>]]

[[Lepidophagy]] is a specialised feeding behaviour in fish that involves eating the scales of other fish.<ref name="fishbase">{{cite web|url=http://fishbase.org/Glossary/Glossary.cfm?TermEnglish=lepidophagy|title=Glossary: Lepidophagy|author=Froese, R. and D. Pauly. Editors.|publisher=FishBase|access-date=2007-04-12}}</ref> Lepidophagy has independently [[evolved]] in at least five freshwater families and seven marine families.<ref name="Janovetz2005">{{cite journal|url=http://jeb.biologists.org/cgi/reprint/208/24/4757.pdf|title=Functional morphology of feeding in the scale-eating specialist ''Catoprion mento''|first=Jeff|last=Janovetz|journal=The Journal of Experimental Biology|volume=208|pages=4757–4768|year=2005|doi=10.1242/jeb.01938|pmid=16326957|issue=Pt 24|s2cid=15566769|doi-access=free|bibcode=2005JExpB.208.4757J }}</ref>

Fish scales can be nutritious, containing a dermal portion and a layer of protein-rich mucus apart from the layers of [[keratin]] and [[Tooth enamel|enamel]]. They are a rich source of [[calcium phosphate]].<ref name="Janovetz2005"/> However, the energy expended to make a strike versus the amount of scales consumed per strike puts a limit on the size of lepidophagous fish, and they are usually [[micropredator|much smaller than their prey]].<ref name="Janovetz2005"/> Scale eating behaviour usually evolves because of lack of food and extreme environmental conditions. The eating of scales and the skin surrounding the scales provides protein rich nutrients that may not be available elsewhere in the niche.<ref name="Pupfish">{{cite journal| author1=Martin, C. | author2=P.C. Wainwright | year=2011 | title=Trophic novelty is linked to exceptional rates of morphological diversification in two adaptive radiations of Cyprinodon pupfish | journal=Evolution | volume=65 | issue=8 | pages=2197–2212 | doi=10.1111/j.1558-5646.2011.01294.x| pmid=21790569 | s2cid=23695342 | doi-access=free | bibcode=2011Evolu..65.2197M }}</ref> ==See also== * [[Age determination in fish]] * [[Animal coloration]] * [[Animal reflectors]] * [[Photonic crystals]] * [[Reptile scale]] * [[Scale (zoology)]] * [[Scale armour]] * [[Snake scales]] * [[Urokotori]] – Japanese fish scaler

==References== {{Reflist|33em}}

==Further reading== * {{cite book |author=Helfman, G.S., B.B. Collette and D.E. Facey |title=The Diversity of Fishes |publisher=Blackwell Science |year=1997 |isbn=978-0-86542-256-8 |pages=33–36}} * {{cite journal | last1 = Schultze | first1 = H.P. | year = 2016 | title = Scales, enamel, cosmine, ganoine, and early osteichthyans | journal = Comptes Rendus Palevol | volume = 15 | issue = 1–2| pages = 83–102 | doi = 10.1016/j.crpv.2015.04.001 | doi-access = free | bibcode = 2016CRPal..15...83S }}

==External links== {{Sister project links|wikt=no|commons=Category:Fish scales|n=no|q=no|s=no|b=no|voy=no|v=no|d=no|species=no}}

* Hydrodynamic aspects of shark scales [https://scholarworks.wm.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1544&context=reports]{{Dead link|date=September 2025 |bot=InternetArchiveBot }} *Fish scales and flow manipulation [https://asknature.org/strategy/scales-manipulate-flow/#.W6tTVvloSHs]

[[Category:Fish anatomy]]